Concept 10
Chromosomes carry genes.

Thomas Hunt Morgan describes his discoveries using fruit flies.

Hi, I'm Thomas Hunt Morgan. In 1904, I started the "Fly" lab at Columbia University to study genetic variations. Many of the important discoveries of genetics and chromosomal inheritance came out of my lab through research using fruit flies. It all started when we found our first useful fruit fly mutation. Flies normally have red eyes but in January, 1910, we found a male with white eyes. We immediately crossed this male with a "wild type" female with red eyes. All the offspring of the first generation were red-eyed! We next crossed males and females from the first generation to each other. The white-eye trait reappeared in the expected 3:1 Mendelian ratio for a recessive trait. However, only the males had white eyes. This suggested that the white-eye trait is carried on the X chromosome. A Punnett square predicts the same results. I used a small "w" for the recessive white and a large "R" for the dominant red. Drosophila mutants are named according to the phenotype of the mutations. Crossing white-eyed males and red-eyed females from the second generation produced equal numbers of offspring with each eye color. Males have white eyes when they inherit the mutant gene on the X chromosome from their mother. Females only show the trait if they inherit mutant genes on both X chromosomes. I called this phenomenon "sex-limited" inheritance, but it became known as "sex-linked" or "X-linked." Within several years, we found more than 80 different mutations in fruit flies. Here are some of the flies from our fly library. I'm a wild type "normal" fly. I have red eyes, a brown body and regular size wings. I'm the famous white eye mutant. I'm a mutant for body color. I have a yellow body. I'm a mutant for wing size. I have miniature wings. I'm a double mutant. I have white eyes and a yellow body. I'm a double mutant. I have white eyes and miniature wings. I'm a double mutant. I have a yellow body and miniature wings. I'm a triple mutant.I have white eyes, a yellow body and miniature wings. Let's bring them all back so that we can compare. I became convinced that these little animals could show us the physical basis of heredity. So began my conversion from an experimental evolutionist to a geneticist.

Thomas Morgan first used milk bottles to grow his fly cultures. And yes, he did get the bottles from his milkman.

Although Mendel had no trouble finding pea plants with lots of different traits, it took a while for Morgan to find a fly with a different trait. Why?